function updateChildren(vnode,newVnode){
    var children=vnode.children || []
    var newChildren=newVnode.children || []

    //遍历现在有 children
    children.forEach(function(child,index){
          var newChild=newChildren[index]
          if (newChild==null) {
              return
          }
          if (child.tag===newChild.tag) {
              //两者tag一样
              updateChildren(child,newChild)
          } else {
              //两者tag不一样
              replaceNode(child,newChild)
          }
    })
}

function replaceNode(){
    //...................
}

